In recent years, terminals provided in components constituting electronic equipment have become smaller under trends of miniaturization and functional enhancement of the electronic equipment. Therefore, in a technical area of electronic mounting, various kinds of anisotropic conductive adhesives have been widely used to easily connect such terminals together. For example, anisotropic conductive adhesives are used to bond integrated circuit chips to a flexible printed circuit board (FPC) or to a glass substrate having an indium-tin-oxide (ITO) electrode circuit formed thereon.
The anisotropic conductive adhesive is a film type adhesive or a paste type adhesive, which includes conducting particles that are dispersed in an insulative resin composition. The anisotropic conductive adhesive is provided between objects so as to bond the objects by heating and pressing. When heated or pressed, a resin in the adhesive flows into a gap between objects so as to seal the space between electrodes facing each other disposed on the objects. Some conducting particles in the resin also flow into the space between the electrodes facing each other so as to achieve an electrical connection therebetween. The anisotropic conductive adhesive is desired to provide a conduction performance in which low resistance (connection resistance) is exhibited between electrodes facing each other in the thickness direction and an insulation performance in which high resistance (insulation resistance) is exhibited between electrodes being adjacently disposed in the lateral direction.
As examples of insulative resin compositions constituting the anisotropic conductive adhesive, epoxy-type thermosetting resin compositions are mainly used. For example, resin compositions containing a curing agent and thermosetting resins such as an epoxy resin and a phenoxy resin are widely used.
The anisotropic conductive adhesive is desired to have high reliability in a connected portion because it is used to connect peripheral components involved in precision apparatuses such as a liquid crystal display (LCD). Therefore, environment-resistant characteristics are desired as well as conduction/insulation performances. The performances are evaluated by a high-temperature-and-high-humidity test or a thermo-cycle test, for example. Note here that the epoxy-type resin composition contained in the anisotropic conductive adhesive has some problems in terms of moisture resistance. That is, since the epoxy-type resin composition includes a hydroxyl group in its molecules, the epoxy-type resin shows high water absorption and poor connections may occur under a high-temperature-and-high-humidity test. Furthermore, since the resin composition has high coefficient of thermal expansion compared with a board material, under a thermo-cycle test, the connection resistance at a connected portion may be increased by the stress occurring due to a difference in coefficient of thermal expansion between a board and the adhesive.
In order to improve the thermal resistance and moisture resistance of the adhesive, Patent document 1 discloses adhesive compositions each having a phenoxy resin, naphthalene-type epoxy resin, and a latent curing agent as critical components. By using a naphthalene-type epoxy resin as an epoxy resin, a glass-transition temperature (Tg) of a cured resin can be raised. Furthermore, in order to reduce the coefficient of thermal expansion and to improve the moisture resistance, a method of adding an inorganic filler to resin compositions has been proposed. Patent document 2 discloses epoxy resin sheet-shaped adhesive compositions each having an epoxy resin, a latent curing agent, an inorganic filler, and polyethersulfone. The disclosed adhesive compositions contain 5 to 900 parts by weight of the inorganic filler per 100 parts by weight of total amount of the epoxy resin, the latent curing agent, and the polyethersulfone.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-315885
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-204324